Glutamine amidotransferases are "complex" enzymes with two different catalytic domains, each contributing to the catalysis of a single biochemical reaction. The family of sixteen enzymes catalyzes the transfer of amide nitrogen from glutamine (glutamine catalytic domain) to a variety of acceptor substrates (acceptor catalytic domain). Crystal structures have been determined for two amidotransferases of de novo purine biosynthesis. The structures of glutamine PRPP amidotransferase (GPATase) and GMP synthetase are prototypes for the two homologous families of glutamine catalytic domains, the Ntn and Triad families, respectively. The acceptor domains of both enzymes also represent homologous enzymes families, the phosphoribosyltransferases and the N-type ATP pyrophosphatases. Results from these first structures of amidotransferases have changed thinking about the enzyme family and led to new direction for future experiments. The most important new discovery of this work has been the means by which Ntn amidotransferases couple catalysis between glutamine and acceptor domains. The enzyme creates a channel for NH/3 between glutamine and acceptor active sites. Signal transduction between catalytic sites is triggered by substrate binding in the acceptor domain and involves formation of the NH/3 channel and activation of the glutamine domain. The proposed studies will investigate control and operation of the NH3 channel in GPATase and determine whether Triad amidotransferases also channel NH/3 between active sites. The product of GPATase is extremely unstable, and there is indirect evidence for channeling to GAP synthetase, the next enzyme of the purine biosynthetic pathway. This possibility will be investigated by structural studies of GAP synthetase and by mutagenesis experiments. The significant observations of the first two amidotransferases will be extended to other members of the enzyme family.